In a display device using a liquid crystal display panel which is a non-radiative type, an electronic latent image formed on the liquid crystal display panel is visualized by providing external lighting means. In the external lighting means, a lighting system is installed at the back or in front of the liquid crystal display panel other than a structure utilizing natural light. Particularly in a display device requiring high intensity, a structure in which a lighting system is provided at the back of a liquid crystal display panel is utilized. This is called a backlight.
The backlight roughly includes a side edge type and a direct type. The side edge type has structure in which a linear light source represented by a cold cathode fluorescent lamp is installed along the side edge of a light transmission plate made of a transparent plate and is often used for a display device requiring thinning, e.g., a small thickness as for a personal computer or the like. In the meantime, in a large-sized liquid crystal display device such as a display device used for a display monitor or a television set, the direct type is often used. A direct backlight has a structure in which a lighting system is installed immediately under the back of a liquid crystal display panel.
Heretofore, for a light source for a liquid crystal backlight of a liquid crystal television, a personal computer and others, a cold cathode fluorescent lamp is generally used. However, in addition to this, there is an external electrode type fluorescent lamp which is attracting attention. The cold cathode fluorescent lamp has a structure in which a pair of electrodes is arranged oppositely inside a sealed glass tube, while the external electrode type fluorescent lamp has a structure in which an electrode is closely attached outside a sealed glass tube. Many advantages such as reducing the cost of connected circuits, simplifying a procedure for manufacture and extending the service life can be expected for the external electrode type fluorescent lamp.
However, the external electrode type fluorescent lamp has a problem that non-radiative length (electrode length) is extended because of its structure. As the area of a frame is being reduced in a recent display device, the reduction of electrode length at the back of the frame means the reduction of the surface area of an electrode in the external electrode type fluorescent lamp and the efficiency of discharge is deteriorated. In other words, if only the surface area of the electrode can be maintained or increased at the same time when the electrode length is reduced, the area of the frame can be reduced without deteriorating the efficiency of discharge.
For the solution of the above-mentioned problem, a configuration in which an electrode part is bent to maintain the surface area of an electrode in place of reducing electrode length is disclosed in a patent document 1, Japanese Patent Laid-open No. 2002-72205. Additionally, a configuration in which the end of a glass tube is made slightly concave to enhance the efficiency of discharge is disclosed in a patent document 2, Japanese Patent Laid-open No. 2004-79268.